Massive Hillock Growth on Cathode Side of Test Structure During Electromigration Experiments

We observed a massive hillock growth on the cathode side of test structures during electromigration experiments. This phenomenon is opposite to the one observd when high self heating thermal gradients are presents. On Al-Si-Ti films, a mass depletion occurs on the anode while hillocks grow on the cathode. The discontinuity of the grain boundary network is responsible for these effects, which are correlated to an enhanced lifetime, On Al-Cu films, hillocks appear at Lhe negative pad - end contact segment junction and their front progagates towards the anode with a velocity proportionnal to current density. As in the cross stipe experiment, the phenomenon results from the difference between the grain boundaries diffusion coefficients in Cu depleted and Cu rich areas.